1. Field of the Invention
The present invention relates to ceramic packages and ceramic packaging materials for large scale integrated electronic circuits, and more particularly to a composition of such ceramic packaging materials adapted to increasing the toughness (resistance to fracture) of such ceramic materials.
2. Description of Related Art
Zirconia particles toughen a ceramic by either impeding the motion of a propagating crack, or absorbing or dissipating its energy. H. Ruf and A. G. Evans, "Toughening by Monoclinic Zirconia", J. Amer. Cer. Soc., 66(55) 328-332 (1983) and N. Claussen and M. Ruhle, "Design of Transformation-Toughened Ceramics", Advances in Ceramics, Amer. Cer. Soc., (3) 137 (1981). The former includes crack bowing and deflection effects. The latter, termed transformation toughening, results from a stress induced martensitic transformation of the zirconia from its tetragonal crystal structure to its monoclinic crystal structure. This transformation is accompanied by a 4% volume increase and a shear strain up to a maximum of 6%. The attainable toughening is dependent upon the volume fraction of transformable tetragonal phase, size and distribution of the zirconia particles, the elastic constraining properties of the matrix and the size of the transformed zone around the crack. Claussen et al. supra; F. F. Lange, "Transformation Toughening", J. Mater. Sci., (17), 235-239 (1982); and A. G. Evans and A. H. Heuer, "Transformation Toughening in Ceramics: Martensitic Transformation in Crack Tip Stress Fields", J. Amer. Cer. Soc., 63 (5-6) 241-248 (1980), and R. McMeeking and A. G. Evans, "Mechanics of Transformation Toughening In Brittle Materials", J. Amer. Cer. Soc., 65(5) 242-245 (1982).
Zirconia toughening has been demonstrated in a variety of crystalline ceramic materials in the last decade. The earliest example, dating back over ten years, was in a two-phase zirconia ceramic consisting of tetragonal zirconia in a matrix of cubic zirconia. This material, described by R. Garvie, R. H. Hannink and R. T. Pascoe, "Ceramic Steel?", Nature, 258(12) 703-704 (1975) was produced by a precipitation heat treatment. Since this original work, a number of workers have demonstrated that the fracture resistance of many crystalline ceramic materials can be increased by incorporating in them particles of tetragonal zirconia, which transform to the monoclinic form when the material is fractured.
Claussen et al. showed that the incorporation of zirconia into an alumina body increased its fracture toughness. Claussen et al. prepared their materials by the usual methods of ceramic forming in which powders of alumina and zirconia were mixed together and then fired to sufficiently high temperatures that the material sintered (densified) to form a monolithic body.
U.S. Pat. No. 4,316,964 of Lange et al. for "Al.sub.2 O.sub.3 /ZrO.sub.2 " describes a zirconia toughened alumina ceramic prepared by using submicron powders of Al.sub.2 O.sub.3 ZrO.sub.2. "The composite powders were ball-milled with methanol and alumina balls in a plastic container and then dried. Densification was achieved by hot-pressing the powders for 2 hours at a temperature selected to obtain small grain size and therefore favor the retention of tetragonal ZrO.sub.2." The pressing temperatures in TABLE 1 were from 1400.degree. to 1600.degree. C. At Col. 5, line 18 it is stated, "The average grain size of the end member compositions which were hot-pressed at 1400.degree. C. was about 2 .mu.m for the Al.sub.2 O.sub.3 and 0.5 .mu.m for the ZrO.sub.2." The ceramic is not a glass ceramic and the pressing temperatures employed are excessive from the point ot view of the packaging of integrated circuits.
Stevens and Evans, "Transformation Toughening by Dispersed Polycrystalline Zirconia", Br. Ceram. Trans. J. Vol. 83, 28-31 (1984) describes transformation toughening of alumina ceramics by volume expansion when tetragonal zirconia transforms to the monoclinic form. It states at page 28, "The phenomenon of transformation toughened ceramics relies on the volume expansion, 3-5% and shear strain .about.7% developed when tetragonal zirconia transforms to the monoclinic form. Toughening of a ceramic host material is attained by retention of the tetragonal zirconia in a metastable state, the phase change to the monoclinic form being initiated by the tensile stress field of an advancing crack. Within a fixed distance of the crack tip, determined by the elastic stress field in its vicinity, any metastable tetragonal zironia will transform and, as a result of the volume expansion and accommodating shear stains, exerts a back stress on the crack. . . . "
U.S. Pat. No. 4,358,516 of Lange, for "Sodium Ion Conductor, Solid Electrolyte Strengthening with Zirconia", describes how the incorporation of transformable tetragonal zirconia could be used to increase the resistance to fracture of a sodium ion conductor solid electrolyte ceramic, .beta.-alumina. For example, the addition of solid grains of metastable tetragonal ZrO.sub.2 with "a grain size less than about 2 .mu.m and has dissolved it in a rare earth oxide such as Y.sub.2 O.sub.3 . . . " (See abstract). The materials are added to the alumina to provide improved fracture toughness.
As in U.S. Pat. No. 4,316,964 Lange, one can use additions of rare earth oxides, such as yttria, to control the formation of zirconia that is stable in the finished material in its tetragonal form.
For example, the addition of 15 vol. % zirconia to .beta."-Al.sub.2 O.sub.3 increases the fracture toughness, K.sub.c, from 3.0 to 3.8 MPam.sup.1/2 and the strength from 147 to 414 MPa. See Stevens et al. supra. Alumina with 7.5 vol. % zirconia shows an increase in K.sub.c from 4.5 to 7 MPam.sup.1/2, and adding 17.5 vol. % zirconia to spinel increased the strength from 200 to 500 MPa. (See Claussen et al. supra.)
Originally used to toughen zirconia ceramics and alumina, the use of transformation toughening has rapidly been accepted as a way of increasing the fracture resistance (toughness) of all sorts of ceramics including oxides, nitrides and carbides. In all these materials, marked increases in toughening have been achieved when the processing conditions have enabled the tetragonal form of zirconia to be retained in the microstructure. However, there is one class of materials in which, despite the incorporation of tetragonal zirconia, true transformation toughening has not been reported, namely glasses and glass-ceramics.
Previous workers, who have attempted to toughen glasses and glass-ceramics by the use of zirconia, have done so by precipitating the zirconia from the glass phase. This is the manner in which zirconia, one of the traditional nucleating aids in the manufacture of glass-ceramics, is known to be formed in its tetragonal form.
For instance, U.S. Pat. No. 4,396,682 of Mohri et al. for "Glazed Ceramic Substrate" describes a ceramic print head with 50-60 wt. % of SiO.sub.2, 10-30 wt. % of CaO and MgO, and 2-6 wt. % of ZrO.sub.2 plus optional materials including the addition of one of BaO, ZnO, PbO, P.sub.2 O.sub.5, B.sub.2 O.sub.3, Na.sub.2 O and K.sub.2 O. The resulting material is a glass which has excellent high temperature stability for use in thermal print heads. The process used involves the heating in air to 1400.degree. C., which is far above an acceptable level of heating for air process since it would be far above the melting point of the metallization. At Col. 3, lines 6-21 it is explained that the ZrO.sub.2 raises the transition point of the glass composition above 2 wt % of ZrO.sub.2 in the material. Above 6 wt % the ZrO.sub.2 becomes an obstacle to the surface smoothness of the material. No mention is made of the tetragonal phase of the zirconia. In view of the temperature of 1400.degree. C. to which the material is heated in the Example, the zirconia is in a solid solution in the glass. Moreover, no mention is made therein of particles of zirconia in the glass.
See also U.S. Pat. No. 4,353,047 where zirconia is added as a nucleating agent.
U.S. Pat. No. 4,234,367 of Herron, Master and Tummala for "Method of Making Multilayered Glass Ceramic Structures Having an Internal Distribution of Copper-based Conductors" describes use of cordierite glass in conjunction with a thermoplastic binder in laminated green sheets. The glass comprises MgO, Al.sub.2 O.sub.3, SiO.sub.2, B.sub.2 O.sub.3, SnO.sub.2, Al.sub.2 O.sub.3, P.sub.2 O.sub.5 and ZrO.sub.2 glass particles in a glass ceramic. The laminate of green sheets is heated to a burnout temperature of 720.degree. to 785.degree. C. Then the laminate is later heated to a crystallization temperature of about 920.degree. to 970.degree. C. Here the composition of the glass particles is quite different from those employed here in view of the presence of P.sub.2 O.sub.5, B.sub.2 O.sub.3 and in some cases SnO.sub.2 and the absence of a stabilizing compound such as yttria plus the absence of teachings of particle size limitations claimed herein.
U.S. Pat. No. 4,301,324 of Kumar, McMillan and Tummala for "Glass Ceramic Structures of Gold, Silver or Copper" describes .beta.-spodumene glass ceramic and cordierite glass ceramic materials. In connection with the cordierite glass. There is no CaO or Y.sub.2 O.sub.3 stabilizing material, but there is CaO with respect to the .beta.-spodumene at Col. 7, lines 61 and 62 and in Table 1, Col. 4. The cordierite glass does contain from 0 to 2.5 wt % of ZrO.sub.2 in Table 1. Glass No. 11 in Table III includes B.sub.2 O.sub.3 plus ZrO.sub.2 but also includes P.sub.2 O.sub.5 and is sintered at 925.degree. C. Glass No. 12 in Table III includes B.sub.2 O.sub.3 but also includes P.sub.2 O.sub.5 and is sintered at 950.degree. C. There is no suggestion of using yttria as a stabilizing agent. There is also no mention of the tetragonal zirconia phase in connection with Glasses No. 11 and 12. The cordierite is of the .mu. form for glass No. 11 and of the .alpha. form for glass No. 12. See Col. 9, lines 17 to 45. It is stated that the ceramic has greater strength than other ceramics. It is stated that it was thought that the enhanced strength was attributable to inclusion of ZrO.sub.2, Col. 9, lines 25 to 28. There is no discussion of the particle sizes of the zirconia. A key distinction of the result of the process of the instant invention from the process of Kumar et al. is that the zirconia is in a solid solution in the glass ceramic. We find that there is no encapsulation of particles of zirconia in the tetragonal phase in the Kumar et al. product.
The objective of this invention is to provide a glass which is transformation toughened through the use of zirconia, which glass is crystallized to a cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) glass-ceramic upon heat treatment. The method of manufacture, a "composite" route in which particles of the glass, having the desired glass-ceramic composition, and the zirconia are mixed together and fired as in a standard ceramic process. It avoids the conventional glass-ceramic manufacturing route which involves melting of the ingredients, and subsequent crystallization to produce the glass-ceramic body. Furthermore, the method enables the size of the tetragonal particles to be kept within a desirable range.
U.S. Pat. No. 4,421,861 of Claussen for "High-Strength and Temperature-Change Resistant Ceramic Formed Body, Especially of Mullite, its Production and Use" describes a zirconia toughened cordierite ceramic made by a reaction process, which involves deriving zirconia from a salt of zirconium. To obtain zirconia in that way the material is sintered at unacceptably high temperatures of from 1300.degree. C., to 1600.degree. C., preferably. Such a high temperature is far higher than an acceptable temperature for formation of VLSI packaging products since it is so high that the copper metallization of the circuits on the package would be destroyed by the heat. In short, the copper would turn to useless puddles on the package. Thus the Claussen et al. process is a very significantly different process from ours. It also produces a much different result.
Ruh et al. "Phase Relations in the System ZrO.sub.2 -Y.sub.2 O sub 3 Contents", Communications of American Ceramic Society, C-190 to C-192, (September 1984), describes use of yttria with zirconia to lower the monoclinic-tetragonal transformation temperature of zirconia, but it does not suggest the use of such material with cordierite.
B. Schwartz "Making High Strength Ceramics", IBM Technical Disclosure Bulletin, Vol. 11, No. 7,848 (December 1968), describes placing the surfaces of a ceramic material in compression relative to the central portions of the article by altering the composition of the outer layers of at least three layers of green ceramic material slightly prior to firing, by adding chromium to alumina. The ceramic materials are used as substrates for microelectronic devices. Obviously this disclosure does not contemplate use of zirconia or the equivalent as the material which provides hardening. In addition it does not suggest the temperature range that is taught here.
In D. J. Green and M. G. Metcalf, "Properties of Slip-Coat Transformation-Toughened .beta."-Al.sub.2 O.sub.3 / ZrO.sub.2 Composites", Ceramic Bulletin, Vol. 63, No. 6 pp. 803-807, and 820 (1984), it is stated at page 805 first full paragraph that "The majority particles are less than 1 .mu.m for both powders but there are some particles as large as 20 .mu.m".
Porter, D. L., Evans, A. G., & Heuer, A. H. Acta MetalVol. 27, p. 1649 (1979) describes toughening of .beta." Alumina and of Zirconia, respectively. None of the prior art suggests the range of sizes of particles of zirconia or hafnia. The temperature range used in forming the hardened ceramic materials is suggested by none of the prior art for forming ceramic, but merely to the formation of ceramics and the Herron et al. U.S. Pat. No. 4,234,367 does not relate to hardening of ceramics, per se.
A number of test methods have been used to measure the fracture toughness of ceramics, and the effect of zirconia additions. One of these is the indentation test method, which is described in detail by Antis et al., "A Critical Evaluation of Indentation Techniques For Measuring Fracture Toughness: I. Direct Crack Measurements", Journal of the American Ceramic Society, 64(9) 533-538, (1981). In this method, a diamond pyramid is pressed, with a known load, into the surface of a material until cracks propagate from the corners of the indentation impression. The length of the cracks so formed for a given load are a measure of the resistance of the material to fracturing (its so-called fracture toughness).
The transformation of zirconia from its tetragonal form to its monoclinic form as a result of the passage of a crack (the basis of transformation toughening) has been shown to lead to an increase in fracture toughness by Garvie et al., "Ceramic Steel?", Nature, (258), pp. 703-704 (1975) and by Clarke and Adar, "Measurement Of The Crystallographically Transformed Zone Produced By Fracture In Ceramics Containing Tetragonal Zirconia", Journal Of The American Ceramic Society 65(6) pp. 284-288 (1982). Garvie et al. measured their materials before and after fracturing by techniques of X-ray diffraction, to show that some of the tetragonal zirconia had transformed to monoclinic zirconia. Clarke and Adar used Raman spectroscopy to show that some of zirconia particles around cracks had transformed from the original tetragonal form to the monoclinic form.
The degree of transformation of zirconia from the tetragonal form to the monoclinic form dictates the attainable toughening. Thus, if only 25% of the tetragonal zirconia in a body is transformed to monoclinic zirconai, the fracture toughness will not exceed 25% of the theoretically possible. Likewise, a material made in such a way that part of the zirconia is already in its monoclinic form, there will be less zirconia available in its tetragonal form for transformation to monoclinic.